Influence of compression arrangements on the structural behaviour of concrete filled steel tubes

The confining pressure to concrete in concrete filled steel tube (CFST) is related to the dilation of concrete core and the hoop stress of steel tube. Due to the passive confinement nature, confinement initiates when the lateral expansion of concrete exceeds that of steel tube. Poor confining stress at initial stage allows the initiation of microcracks in concrete and weakens the ultimate strength, especially in CFSTs with high strength concrete. This paper aims to improve the initial confinement by changing the load arrangements and compressive behaviours of CFST columns under different axial load conditions are investigated. In total, forty-four CFST specimens are tested under different arrangements of axial compression, namely CFST, steel tube confined concrete (STCC) and one-side concrete loaded CFST (CLCFST). Test results indicate that the ultimate strength of the columns with high strength concrete is improved but the initial stiffness is reduced when the axial compression is only applied to the concrete core at one end (CLCFST) or two ends of the column (STCC). Finite element (FE) models are subsequently established, and then validated against the test results. The validated FE models are used to carry out a comprehensive parametric study to investigate the axial stress distribution in CFST columns with different load arrangements. It is found that the load introduction between concrete and steel tubes is mainly affected by the diameter-to-thickness ratio of steel tube and the friction coefficient.